1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Rotating field sensors are used not only to detect the rotational position of an object but also to detect a linear displacement of an object. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in conjunction with the rotation or linear movement of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position or linear displacement of the object is thus detected.
There has been known a rotating field sensor that has two bridge circuits (Wheatstone bridge circuits) as shown in U.S. Pat. No. 6,943,544 B2. In this rotating field sensor, each of the two bridge circuits includes four magnetoresistive elements (hereinafter referred to as MR elements) serving as magnetic detection elements. Each of the bridge circuits detects the intensity of a component of the rotating magnetic field in one direction, and outputs a signal that indicates the intensity. The output signals of the two bridge circuits differ in phase by ¼ the period of the output signals of the bridge circuits. The angle that the direction of the rotating magnetic field forms with respect to a reference direction is calculated based on the output signals of the two bridge circuits.
Various types of rotating field sensors have heretofore been proposed that have a pair of magnetic detection elements for outputting a pair of detection signals having a phase difference of 180°. For example, JP-A-2009-186410 describes a rotation detecting apparatus including an encoder and a sensor unit. In the rotation detecting apparatus, the encoder has a portion to be detected that is arranged concentrically to the center of rotation of a rotating member. The magnetic property of the portion to be detected alternates in the circumferential direction. In the rotation detecting apparatus, the detecting part of the sensor unit includes a pair of magnetic detection elements of the same type. The magnetic detection elements are arranged so as to coincide in phase in the circumferential direction of the encoder and differ in phase by 180° in the direction of flow of magnetic flux.
JP-A-2009-186410 describes that the output signals of the pair of magnetic detection elements can be input to a differential line receiver to eliminate the effect of electrical noise that the transmission signals in the cable undergo from outside.
JP-A-2005-315696 describes a rotation angle detecting apparatus including a magnet that rotates with the rotation of a rotating body, and first and second groups of magnetic detection elements. In the rotation angle detecting apparatus, the magnet is formed in a cylindrical shape and is magnetized to two poles, or an N pole and an S pole, in parallel. The first and second groups of magnetic detection elements each include four Hall devices serving as the magnetic detection elements, which are arranged at intervals of 90° around the axis of rotation of the magnet. The four Hall devices H1 to H4 of the first group of magnetic detection elements and the four Hall devices H5 to H8 of the second group of magnetic detection elements are alternately arranged at intervals of 45°. The four Hall devices H1 to H4 of the first group of magnetic detection elements output signals of sinusoidal waveform with a phase difference of 90° from each other. Similarly, the four Hall devices H5 to H8 of the second group of magnetic detection elements output signals of sinusoidal waveform with a phase difference of 90° from each other.
The rotation angle detecting apparatus described in JP-A-2005-315696 generates difference data from the output signals of two Hall devices that lie at an interval of 180°, and detects the rotation angle of the rotating body based on the difference data. Specifically, the rotation angle detecting apparatus generates difference data H1-H2, H3-H4, H6-H5, and H8-H7 on respective four pairs of two Hall devices at an interval of 180°, i.e., H1 and H2, H3 and H4, H5 and H6, and H7 and H8. A rotation angle θ1 that is detected by the first group of magnetic detection elements is calculated from the difference data H1-H2 and the difference data H3-H4. A rotation angle θ2 that is detected by the second group of magnetic detection elements is calculated from the difference data H6-H5 and the difference data H8-H7.
JP-A-2005-315696 describes that the generation of the difference data from the output signals of two Hall devices at an interval of 180° can cancel the difference between the center of the magnet and the center of the Hall array of the eight Hall devices. JP-A-2005-315696 further describes that the rotation angle detected by the first group of magnetic detection elements and that detected by the second group of magnetic detection elements are compared to determine the presence or absence of the occurrence of an abnormal condition.
The magnetic detection elements of a rotating field sensor sometimes undergo not only the rotating magnetic field to detect but also a magnetic field other than the rotating magnetic field to detect. Such a magnetic field other than the rotating magnetic field will hereinafter be referred to as noise field. Examples of the noise field include a leakage magnetic field from a motor and the magnetism of the earth. When a noise field is thus applied to the magnetic detection elements, the magnetic detection elements detect a composite magnetic field resulting from a combination of the rotating magnetic field and the noise field. If the rotating magnetic field to detect and the noise field differ in direction, the angle detected by the rotating field sensor includes some error. For example, suppose that the rotating magnetic field to detect, in terms of magnetic flux density, has a magnitude of 20 mT, the noise field has a magnitude equivalent to the earth's magnetism, or 0.05 mT, and the direction of the noise field is orthogonal to that of the rotating magnetic field to detect. In such a case, the direction of the composite magnetic field is different from that of the rotating magnetic field to detect by 0.14°. As a result, the angle detected by the rotating field sensor includes an error of 0.14°. This shows that if, for example, an angle accuracy (resolution) of 0.1° is required of the angle to be detected by the rotating field sensor, even the earth's magnetism can be an extremely large noise source.
To reduce such an error resulting from the noise field in the angle detected by the rotating field sensor, a possible measure is to cover the magnetic detection elements and the magnet that generates the rotating magnetic field with a magnetic shield integrated with the rotating field sensor. If the source of the noise field is known, a magnetic shield can be provided between the source of the noise field and the magnetic detection elements. Such measures, however, have the drawbacks of making the design of the rotating field sensor including the magnetic shield large in scale, increasing the cost of the rotating field sensor, and placing various constraints on the assembly steps and on the installation of the rotating field sensor.
According to the rotation detecting apparatus described in JP-A-2009-186410, electrical noise produces errors of the same sign in the respective output signals of the pair of magnetic detection elements. Therefore, determining the difference between the output signals of the pair of magnetic detection elements can reduce the error in the detected angle resulting from the electrical noise. In the rotation detecting apparatus described in JP-A-2009-186410, however, a noise field produces errors of opposite sign in the respective output signals of the pair of magnetic detection elements. Determining the difference between the output signals of the pair of magnetic detection elements therefore cannot reduce the error in the detected angle resulting from the noise field.
According to the rotation angle detecting apparatus described in JP-A-2005-315696, the generation of difference data from the output signals of two Hall devices that lie at an interval of 180° can reduce the error in the detected angle resulting from the noise field. To obtain a detected angle, i.e., an angle θ1 or θ2, however, the rotation angle detecting apparatus needs at least four magnetic detection elements (Hall devices) that are arranged at intervals of 90° around the axis of rotation of the magnet. The rotation angle detecting apparatus described in JP-A-2005-315696 thus has a drawback that its application is limited to cases where the four magnetic detection elements can be arranged at intervals of 90°.